CT of the iliopsoas compartment: value in differentiating tumor, abscess, and hematoma

The iliopsoas compartment is an extraperitoneal space defined by the iliopsoas fascia; its major components include the iliacus, psoas major, and psoas minor muscles. Distinction of neoplastic, inflammatory, and hemorrhagic conditions of the iliopsoas compartment is often difficult, whether based on clinical evaluation or CT findings. The purpose of this study was to determine specific CT features that might help differentiate these three abnormalities.

We retrospectively reviewed CT scans of 44 patients who had abnormalities of the iliopsoas compartment (15 neoplasms, 21 abscesses, and eight hematomas). Final diagnoses were established by surgical biopsy (n = 14), percutaneous biopsy (n = 18), or clinical evidence (n = 12). Diagnoses were correlated with 11 CT features to determine findings that could be used to differentiate the three abnormalities. These included enlargement of the iliopsoas muscles; extent, margination, and attenuation of the lesion; presence of gas, calcification, bone destruction, fat infiltration, and fascial disruption; and associated fluid or adenopathy. The radiologist who interpreted the scans had no knowledge of the clinical findings.

The most reliable CT features for each condition were as follows: irregular margins, 67% sensitive, 52% specific, and 57% accurate for neoplasms; low attenuation, 100% sensitive, 43% specific, and 70% accurate for abscesses; and diffuse involvement of the entire muscle, 88% sensitive, 78% specific, and 80% accurate for hematomas. Combinations of individual CT features did not improve the ability to differentiate the three conditions.

Our results show that the efficacy of CT, when scans are interpreted without knowledge of the clinical history, is poor for differentiating iliopsoas neoplasms, abscesses, and hematomas.